Geoff Marcy, a professor of astronomy at Berkeley, talks to The PBS News Hour about earth-like planets.

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“Earth-like,” as used by Marcy, means a rocky planet orbiting in the Goldilocks zone (neither too close nor too far from its star), and thus where liquid water could potentially flow on the surface. One in five stars, according to a recent Kepler satellite data sample of 150,000 stars, are estimated to have a planet that fits this definition–which means that there are billions of such planets in our Milky Way galaxy alone. Mars is an example of an “earth-like” planet.

What percentage of these planets has an atmosphere, a moon stabilizing its rotation, a magnetic field deflecting solar winds, abundant water, and life is unknown.

And let’s not forget the potentially life-harboring moons that may number in the billions as well. At the bottom of this page is the image that the Huygens probe took in 2005 of the surface of Titan, Saturn’s largest moon (image source: NASA via Wikipedia). It’s rocky and has water, in the form of ice, but wouldn’t qualify as “earth-like” because water does not gather or flow on the moon’s surface (though methane does; there’s a greenhouse effect at work on Titan).

And if Wikipedia summarizes the state of current research accurately, Titan has methane lakes and perhaps methanogenic life. If it has any form of life, however different from our own, it’s certainly more “earth-like” than, say, Mars or our moon, though out of the Goldilocks zone:

In 2005, astrobiologistChris McKay argued that if methanogenic life did exist on the surface of Titan, it would likely have a measurable effect on the mixing ratio in the Titan troposphere: levels of hydrogen and acetylene would be measurably lower than otherwise expected.[136]

In 2010, Darrell Strobel, from Johns Hopkins University, identified a greater abundance of molecular hydrogen in the upper atmospheric layers of Titan compared to the lower layers, arguing for a downward flow at a rate of roughly 1025 molecules per second and disappearance of hydrogen near Titan’s surface; as Strobel noted, his findings were in line with the effects McKay had predicted if methanogenic life-forms were present.[136][139][140] The same year, another study showed low levels of acetylene on Titan’s surface, which were interpreted by McKay as consistent with the hypothesis of organisms consuming hydrocarbons.[139] Although restating the biological hypothesis, he cautioned that other explanations for the hydrogen and acetylene findings are more likely: the possibilities of yet unidentified physical or chemical processes (e.g., a surface catalyst accepting hydrocarbons or hydrogen), or flaws in the current models of material flow.[141] Composition data and transport models need to be substantiated, and per Occam’s razor, a physical or chemical explanation is preferred a priori over one of biology (given the simplicity of chemical catalysts versus the complexity of biological forms). Even so, McKay noted that the discovery of a catalyst effective at 95 K (−180 °C) would still be significant.[129]

As NASA notes in its news article on the June 2010 findings: “To date, methane-based life forms are only hypothetical. Scientists have not yet detected this form of life anywhere”.[139] As the NASA statement also says: “some scientists believe these chemical signatures bolster the argument for a primitive, exotic form of life or precursor to life on Titan’s surface.”[139]

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At the end of Arthur C. Clark’s novel, 2001:A Space Odyssey, the astronaut David Bowman’s last words, before entering the monolith, are, “My God–it’s full of stars!”